Researchers at University of California, Berkeley have built nanolasers on …

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Sometimes I wonder if some groups structure their articles via conversations that go something like this:

Does the paper include a laser? Yes

Do we have the word nano in there? Yes!

Can we put them together? Yes!

On silicon? YESSSS!

Awesome! Now we have the nanolasers. Quick, zip the file up and send it to Nature.

A paper that may turn out to be important turned up in Nature Photonics, and it hits all the keywords, including lasers, nano, and the phrase optical interconnects. It also does so in such a way that may have Intel reaching for its wallet. But don't expect to find yourself firing up a computer with a laser bus in the near future.

Here is a pop quiz: what is missing from the following list; gallium, arsenide, indium, phosphorous, aluminum, and nitrogen? If you answered silicon, you may now bask in the glory of an anonymous victory. You see, the elements in the list are used to make semiconductor-based lasers. But, the semiconductor industry is silicon-based, so having silicon missing from that list is a big deal.

What it means is that it is actually pretty difficult to make a semiconductor laser based on good old complementary metal oxide-semiconductor (CMOS) processes used in traditional semiconductor manufacturing. Indeed, it is so difficult that Intel has gone the route of making Raman lasers out of silicon. I am not going to explain what a Raman laser is, but suffice it to say that the list of things I would rather be doing than developing Raman lasers is rather large and includes such tasks as cleaning septic tanks and drilling for water in the Atacama desert.

Now, if you were me—someone who knows nothing about the details of material deposition—you would just say: "harden up and start depositing layers of gallium arsenide on your silicon chip." After everyone in the room had wiped the tears of laughter from their eyes, they would gently explain that you could do that, but the spacing between atoms in silicon was rather different from the spacing in gallium arsenide crystals. They would then continue on to explain that the two materials are grown at different temperatures, so the structures that you had so carefully manufactured with your CMOS process would end up resembling a butter sculpture on a Bunsen burner. They might even end the conversation with something along the lines of "grow away."

Into this rather bleak picture step our heroes: Chen and colleagues from University of California at Berkeley. They followed the final bit of advice from their materials science colleagues and attempted to grow gallium arsenide and Indium gallium arsenide on a silicon substrate. But they weren't looking to get a clean and complete layer. Instead, they have a growth recipe that results in well separated pillars that are about 500nm in diameter.

At this size, each pillar is small enough that the mismatch between the atomic spacing of the different materials can't create enough stress to fracture the pillar—stress builds up as a function of the layer area and depth. So they had single crystalline pillars of a material with good laser gain, sitting on top of a silicon wafer. Better yet, the growth conditions were such that, had the wafer been through a CMOS process first, the electronic circuitry would have survived.

Of course, a crystal by itself is not a laser and, at first sight, you wouldn't expect this to lase. Why not? At the interface between the silicon and gallium arsenide, the refractive index difference is really small, which means that most of the light hitting that interface disappears out of the laser and into the silicon. Since a laser needs some of this light fed back to it, you would expect the crystals to behave like an LED. But, in fact, the nanocrystal pillars allow light to bounce around inside in a number of different ways, so there are a set of light paths that involve hitting the interface between the gallium arsenide and silicon at an oblique angle, where most of the light is reflected.

This is one of the joys of nature: even though you didn't think of that particular mode of operation, if the mode is available, the laser will use it whether you want it to or not. What surprised me was that the researchers made rather a big deal of this. I mean, seriously. Lasers work on whatever mode doesn't lose too much light—lasers created from powders are a great example of this. Just because you didn't anticipate that particular mode doesn't mean that it is an important detail.

But the laser works, hooray, get my electronic laser interconnects right now. "I want a 10Gb/s bus thank you very much," I hear you shout. But not so fast, sunshine. You see, there are a couple of minor problems. First, the wavelength of the laser is one that is absorbed by silicon; adding phosphorus would shift the wavelength to one that isn't absorbed. I suspect that, since they chose to publish what they had, that they tried to grow crystals with phosphorous in them and failed.

The second problem is one of waveguiding. They are going to have to develop a process that involves laying down the CMOS electronics, then a layer that has optical waveguides, which must connect to the electronics at the right locations. Remember, the feature size of the electronics is 45nm, while the laser is 500nm in diameter, so lining things up won't be trivial. On top of that, they will have to deposit their nanolaser crystals in exactly the right locations. And, finally, you have to put some electrical connections on the nanocrystal—the researchers got their lasers going by exciting them with another laser. All these fixes seem a bit involved really.

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Chris Lee
Chris writes for Ars Technica's science section. A physicist by day and science writer by night, he specializes in quantum physics and optics. He Lives and works in Eindhoven, the Netherlands. Emailchris.lee@arstechnica.com